1. Field of the Invention
The present invention relates to primary cells and more particularly to an improved means for venting sealed primary cells when incordinate pressure is generated therein.
2. Prior Art
A common type of seal primary cell comprises a relatively deep, tubular envelope, usually drawn from metal, and a separate cover and/or disc which is used to close the envelope after the latter is filled with electrolyte, separators, and other materials depending upon the nature of the cell. Both LeClanche and alkaline cells have been formed in this manner. Although such cells have been produced in a vast array of shapes and sizes, perhaps the most common type of cell constitute the relatively small, cylindrical cell often broadly termed a "flashlight battery". Of course, primary cells are used in countless other types of apparatus, and are formed in many different sizes and capacities.
The construction of such cells is far from simple, and must meet exacting demands with respect to both insulating and sealing capacity. In particular, the cell construction must not only be economical, but must be liquid-tight to prevent the escape of the cell contents.
Generally speaking, a sealed primary cell will theoretically operate at a relatively low internal pressure, which seldom exceeds approximately 1/2 atmospheres (absolute). In use, however, this figure is often exceeded. One particularly common cause of severe overpressure is the inadvertent reversal of a cell in an application wherein it is series-connected with two or more other cells. In such a case, the potential of the correctly aligned cell overcomes that of the reversed cell and causes the reversed cell to charge rapidly. The cell electrolyte then "outgases", gas being developed much more rapidly than it can be reabsorbed by the cell contents. As a result pressure rapidly builds up within the cell and, if not relieved, may cause the cell to burst, dispersing the cell electrolyte and possibly damaging the enclosure in which the cells are disposed.
For the foregoing reasons, it has become commonplace to provide venting or pressure relief mechanisms in sealed cells. Such venting mechanisms ordinarily allow gas to escape from the cell under overpressure conditions along a predetermined path, and are intended to relieve gas pressure before the cell bursts. While in principal the provision of a pressure relief vent is straightforward, due to the small size and relatively low price of the more popular size cells, the design of appropriate venting mechanisms is difficult.
Basically, three types of venting systems have evolved for use in sealed cells. With one approach, a spring loaded valve is provided, the valve being biased by the spring with a pressure which is overcome during outgasing conditions, temporarily opening the valve against the pressure of the spring. With another approach, a venting passageway is covered by a resilient member, such as a plastic washer. The resilient member can be temporarily deformed by excess pressure in the cell, forcing it away from the venting opening and allowing the gas to escape from within the cell.
Finally, a currently popular approach with small size sealed cells is providing a weakened section or diaphragm which is ruptured by unduly high pressure within the cell. In one version of this approach the diaphragm is simply stretched to the bursting point by internal pressure; in another, the diaphragm is urged outwardly against a point anvil or the like which punctures the diaphragm allowing the pent-up gases to escape. The latter two approaches have found favorite with manufacturers of sealed cells due at least in part to their simplicity which allows them to be easily miniaturized for use with smaller cells.
One of the reasons of the popularity of the diaphragm venting system is that it can be injection molded in one piece in one operation and then placed into the cell. However, there are problems in using the diaphragm of the prior art in that the newer harder plastics cannot be injection molded to form a diaphragm thin enough to burst at the pressure required in a sealed primary cell and once the diaphragm is burst, it cannot be resealed and the contents of the cell can leak.
Accordingly, it would be highly desirable to be able to provide a diaphragm venting system in which the disc could be made of a newer harder plastic, which improve the characteristics of cell, and still be injection molded.
It also would be highly desirable to provide a diaphragm type venting system in which the diaphragm could, at least in the lower range of high pressure situations, readily release the excess gas and then be resealed, preventing leakage, but which would be capable under excessive high pressure to burst fully and provide a safe cell.